Flexbox all the time (Drop IE9 support and remove $enable-flex option) (#21389)

* remove the $enable-flex variable option

* remove bootstrap-flex.css dist file and it's grunt task

* remove the separate flex css file for docs; it's all the same now

* remove flexbox docs (porting some to the main grid docs in next commit)

* clean up few grid docs bits to simplify copy, start to mention flexbox

* port relevant flexbox-grid.md content to grid.md

- clean up mixins
- update how it works section
- bring over sizing and alignment sections

* remove the $enable-flex from the options.md page

* update lead paragraph to mention flexbox

* update migration to mention loss of ie9 support

* remove mention of flexbox dist file

* clarify IE support

* making a note

* remove flexbox variant mentions from component docs

- updates docs for media object, navs, list group, and cards to consolidate docs
- no more need to callout flexbox variants since it's now the default

* remove $enable-flex if/else from sass files

* remove flex dist files

* update scss lint property order to account for flex properties

* linting

* change to numberless classes for autosizing, wrap in highlighting div

* bump gruntfile and postcss to ie10

* redo intro sections

* rearrange

* phew, redo hella grid docs

- rearrange all the things
- consolidate some bits

* remove reference to flexbox mode

* more border action for demo

* Make some changes to the .card's in .card-deck's to ensure footers align to the bottom

docs/getting-started/browsers-devices.md

@@ -94,7 +94,7 @@ Similarly, the latest versions of most desktop browsers are supported.
         <th scope="row">Windows</th>
         <td class="text-success">Supported</td>
         <td class="text-success">Supported</td>
-        <td class="text-success">Supported</td>
+        <td class="text-success">Supported, IE10+</td>
         <td class="text-success">Supported</td>
         <td class="text-success">Supported</td>
         <td class="text-danger">Not supported</td>
@@ -109,9 +109,9 @@ Unofficially, Bootstrap should look and behave well enough in Chromium and Chrom
 
 For a list of some of the browser bugs that Bootstrap has to grapple with, see our [Wall of browser bugs]({{ site.baseurl }}/browser-bugs/).
 
-## Internet Explorer 9 & 10
+## Internet Explorer
 
-Internet Explorer 9 & 10 are also supported, however, please be aware that some CSS3 properties and HTML5 elements are not fully supported.
+Internet Explorer 10+ is supported, however, IE9 down is not. Please be aware that some CSS3 properties and HTML5 elements are not fully supported.
 
 <div class="table-responsive">
   <table class="table table-bordered table-striped">
@@ -144,11 +144,11 @@ Internet Explorer 9 & 10 are also supported, however, please be aware that some
 
 Visit [Can I use...](http://caniuse.com/) for details on browser support of CSS3 and HTML5 features.
 
-## Supporting Internet Explorer 8
+## Supporting Internet Explorer 8-9
 
-As of v4, Bootstrap no longer supports IE8. **If you require IE8 support, we recommend you use Bootstrap 3.** It's still supported by our team for bugfixes and documentation changes, but no new features will be added to it.
+As of v4, Bootstrap no longer supports IE8 or IE9. **If you require IE8-9 support, we recommend you use Bootstrap 3.** It's still supported by our team for bugfixes and documentation changes, but no new features will be added to it.
 
-Alternatively, you may add some third party JavaScript to backfill support for IE8 to Bootstrap 4. You'll need the following:
+Alternatively, you may add some third party JavaScript to backfill support for IE8-9 to Bootstrap 4. You'll need the following:
 
 * [The HTML5 shiv](https://en.wikipedia.org/wiki/HTML5_Shiv)
 * [Respond.js](https://github.com/scottjehl/Respond)